This invention relates to apparatus for measuring the hardness of rubber, or a hardness meter, of the type wherein a needle or an indentor is urged against the surface of a sample of rubber and the penetration depth of the needle is measured to determine the hardness of the sample.
The hardness meter of the type just described is constructed such that when a pressing plate having an opening for passing the needle is urged against the surface of the sample, a needle having a prescribed dimension determined by a regulation, for example JIS (Japanese Industrial Standard) K 6301, is urged against the surface of the sample, that the spring pressure is proportioned such that when the displacement of the needle is zero a definite load will be applied to the needle so that the distance between the pressing plate and the end of the needle is proportional to the load applied to the end of the needle, and that the distance over which the needle is pressed back by the surface of the sample is graduated in terms of hardness which is inversely proportional to the hardness.
In operation, the tester is maintained upright and the pressing plate is urged against the surface of the sample while maintaining the needle at right angles with respect thereto so as to determine the hardness of the sample by reading the scale. According to said JIS, the scale is to be read immediately after contacting the pressing plate against the sample. According to (American Standard of Testing Materials) the scale is to be read within one second after contacting the pressing plate or the maximum value should be read, whereas according to DIN (Deutsche Industrie-Norm) the scale should be read after 3 seconds after contacting the pressing plate.
The shape and thickness of the sample are also prescribed by regulations.
In the hardness test of rubber there are the following problems.
1. Since the hardness meter is manually contacted against the surface of the rubber sample the speed and force of urging the needle are not constant thus degrading the test result.
2. When reading the scale, as the pointer moves slowly immediately after contacting the hardness meter against the surface of the sample, the accuracy of the reading varies depending upon the response speed and the skill of the reader.
Thus, measurement errors and personal errors are caused by the method of penetrating the needle and the time of reading the displayed value.